Electrostatic copying machines of the type which utilize strip-type photoconductors, including a surface adapted to carry a developable electrostatic latent image, generally utilize dry developer materials that normally include a toner material and a carrier material. For development purposes, the toner material is charged to a polarity opposite to that of the charge of the latent image on the photoconductor. As a consequence, when the developer material is brought into contact with the image bearing surface of the photoconductor, toner material adhers to the image, thereby rendering the same visible. Accordingly, copying machines of the aforesaid type are generally provided with means for contacting the image bearing surface of the photoconductor with developer material from a readily available working supply of the same.
One of the known types of apparatus available for developing latent images is a magnetic-brush developer. For use with such developers, the carrier particles of the developer material are made of a magnetizable material, such as iron. A typical magnetic-brush developer includes a rotating applicator immersed in a working supply of developer material, and a moving magnet which cooperates with the applicator for forming a magnetic field in a space between the applicator and the magnet. Since a photoconductor bearing the electrostatic latent image is also disposed in the field space, with the image surface facing the applicator as the applicator urges developer material into the field space, the field entrains the developer material disposed on the surface of the applicator. As applicator and magnet move in different directions relative to one another, the entrained developer material brushes the image bearing surface of the photoconductor, thereby developing the latent image. The developer material on the applicator is then returned directly to the working supply of developer material, mixed with the same and recycled.
To prevent the working supply of developer material from becoming gradually useless due to wear and tear of the carrier material content and continuous dissipation of the toner material content, the supply must be rejuvenated from time-to-time. This is normally done by adding developer material, including toner and/or carrier material, from a replenishing supply, directly into the working supply of developer material and mixing the added material with the working supply along with the developer material returned to the working supply from the applicator.
In order to determine the amount of developer material which must be added from the replenishing supply, it is necessary to determine the state of the developer material before, during and after the replenishment. This is accomplished through use of a toner density controller which uses the reflective properties of the development material in order to determine its condition. One problem that has been encountered in the use of this type of toner density controller is that the fines in the developer material tend to interfer with the ability to measure reflection as such fines tend to cloud the area between the toner density controller and the developer material.
It has been found that the problem may be alleviated by applying a small voltage to the toner density controller which is opposite in charge to that of the sump containing the toner material. This creates a large differential voltage between the controller and the development material because the sump is polarized with an opposite polarity relative to the controller. In this way, the toner material is repelled away from the toner density controller and the light directed from the latter, as well as the light reflected from the surface of the development material, will not be interferred with by dust or fines usually given off from the toner material.